Circuits and methods for current sensing

ABSTRACT

A circuit includes a first resistor, a second resistor, a voltage follower and a current mirror. The first resistor converts a current flowing through the first resistor to a voltage drop between positive and negative sides of the first resistor. The second resistor is coupled to the negative side of the first resistor. The voltage follower is coupled to the positive side of the first resistor via a non-inverting terminal, and coupled to the negative side of the first resistor through the second resistor via an inverting terminal to cause a voltage at the inverting terminal to follow a voltage at the non-inverting terminal. The current mirror is coupled to the voltage follower to provide a sensing current proportional to the current flowing through the first resistor.

RELATED APPLICATION

This application is a continuation of the co-pending U.S. applicationSer. No. 12/406,269, titled “Circuits and Methods for Current Sensing,”filed Mar. 18, 2009, which itself claims priority to U.S. ProvisionalApplication No. 61/072,617, filed Apr. 1, 2008, all which is herebyincorporated by reference.

BACKGROUND

Current sensing circuits are included in electric circuits for detectingcurrent conditions. For example, current sensing circuits implemented inDC/DC converters sense a load current such that the load current can becontrolled. Generally, DC/DC converters utilize an internal resistiveelement (e.g., an internal resistor or inductor direct currentresistance (DCR)) to sense the load current. However, this approach maybe inadequate for highly accurate applications since the internalresistive element is process-dependent and meanwhile suffers from athermal coefficient different with discrete resistive elements inexternal circuits. In other circumstances, regular voltage amplifiersare utilized in current sensing circuits to enlarge the voltage of theinternal resistive element and convert the enlarged voltage into acurrent for subsequent signal processing. However, regular voltageamplifiers tend to impose stability concerns on current sensing.

SUMMARY

A circuit includes a first resistor, a second resistor, a voltagefollower and a current mirror. The first resistor converts a currentflowing through the first resistor to a voltage drop between positiveand negative sides of the first resistor. The second resistor is coupledto the negative side of the first resistor. The voltage follower iscoupled to the positive side of the first resistor via a non-invertingterminal, and coupled to the negative side of the first resistor throughthe second resistor via an inverting terminal to cause a voltage at theinverting terminal to follow a voltage at the non-inverting terminal.The current mirror is coupled to the voltage follower to provide asensing current proportional to the current flowing through the firstresistor

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention will be apparent from the followingdetailed description of exemplary embodiments thereof, which descriptionshould be considered in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram of a current sensing circuit according toone embodiment of the present invention.

FIG. 2 is schematic diagram of an over current protection (OCP) circuitaccording to one embodiment of the present invention.

FIG. 3 is a schematic diagram of a load line control circuit accordingto one embodiment of the present invention.

FIG. 4 a schematic diagram of a current monitor circuit according to oneembodiment of the present invention.

FIG. 5 is a flow chart of a method for current sensing according to oneembodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentinvention. While the invention will be described in conjunction with theembodiments, it will be understood that they are not intended to limitthe invention to these embodiments. On the contrary, the invention isintended to cover alternatives, modifications and equivalents, which maybe included within the spirit and scope of the invention as defined bythe appended claims.

FIG. 1 illustrates a schematic diagram of a current sensing circuit 100according to one embodiment of the present invention. The currentsensing circuit 100 includes a current mirror 101, an amplifier 103, atransistor 105, an external resistor 107, and a sense resistor 109, inone embodiment. The current sensing circuit 100 is used to sense thecurrent flowing through an electronic component. For example, theelectronic component can be an inductor 111 in a DC/DC converter.

To sense the inductor current, the sense resistor 109 is coupled inseries with the inductor 111 at the left side of a capacitor 115. As theinductor current flows through the sense resistor 109, a sensing voltageVsense equal to I_(L)*Rsense is produced across the sense resistor 109,where I_(L) is the inductor current and Rsense is the resistance of thesense resistor 109. In other words, a voltage drop Vsense equal toI_(L)*Rsense is produced between positive and negative sides of thesense resistor 109. Furthermore, the positive side of the sense resistor109 is coupled to the non-inverting terminal of the amplifier 103, inone embodiment. The negative side of the sense resistor 109 is coupledto the inverting terminal of the amplifier 103 through the externalresistor 107, in one embodiment.

The output terminal of the amplifier 103 is coupled to the gate of thetransistor 105. The drain of the transistor 105 is coupled to thecurrent mirror 101, and the source of the transistor 105 is coupled tothe inverting terminal of the amplifier 103. As such, the transistor 105is used as a negative feedback circuit for the amplifier 103, and theamplifier 103 and the transistor 105 constitute a voltage follower 130.In other words, the voltage at the inverting terminal of the amplifier103 is forced to be equal to the voltage at the non-inverting terminalof the amplifier 103 by the negative feedback circuit. As such, thevoltage drop applied on the external resistor 107 is also substantiallyequal to Vsense. Therefore, a current substantially equal to Vsense/Rgis formed to flow from the negative feedback circuit to the externalresistor 107, where Rg is the resistance of the external resistor 107.As such, the current flowing through the transistor 105 is alsosubstantially equal to Vsense/Rg. The current flowing through thetransistor 105 is further duplicated by the current mirror 101 andoutput on an output line 113 as a sensing current Isense, in oneembodiment. Accordingly, the sensing current Isense can be given by

Isense=Vsense/Rg=I _(L) ×Rsense/Rg.  (1)

In the example of FIG. 1, the sense resistor 109 is coupled between theinductor 111 and the capacitor 115. Alternatively, the sense resistor109 can be coupled in series with the inductor 111 at the right side ofthe capacitor 115. In this instance, a load current I_(load), e.g.,which represents a current flowing through a load of the DC/DC converteris equal to the inductor current I_(L) plus Isense. Given byRsense/Rg>>1, practical implementation of Isense can be negligible.Thus, I_(load) is approximately equal to I_(L). Accordingly, the sensingcurrent Isense can be given by

Isense=Vsense/Rg=I _(load) ×Rsense/Rg.  (2)

In practical implementation, the current mirror 101, the amplifier 103and the transistor 105 can be integrated together as a sense amplifier120. The sense amplifier 120 can interface with the external resistor107 and the sense resistor 109 respectively via a CSP pin and a CSN pin.In other words, the voltage follower 130, which includes the amplifier103 and the transistor 105, is coupled to the CSP pin and the CSN pinvia the non-inverting and inverting terminals of the voltage follower130 (i.e., the non-inverting and inverting terminals of the amplifier103) respectively, as shown in FIG. 1. As such, a gain of the senseamplifier 120 can be determined by external components (the externalresistor 107 and the sense resistor 109). Due to the configurability andstability of the external resistor 107 and the sense resistor 109, thegain of the sense amplifier 120 is user adjustable and relatively timestable, in one embodiment. In one embodiment, because both the externalresistor 107 and the sense resistor 109 are placed externally, accuracyissue caused by the different thermal coefficient of internal resistiveelements can be resolved. In one embodiment, the external resistor 107and/or the sense resistor 109 can be temperature-dependent (thermistors)such that the thermal drift of current sensing element (Inductor DCRsensing) within the DC/DC converter can be compensated to maintain thestability of the sensing current Isense.

Advantageously, besides the sensing current Isense on the output line113, additional sensing current outputs can be obtained from the currentmirror 101. These sensing current outputs can be used in a variety ofapplications, such as over current protection, load line control andcurrent monitor.

FIG. 2 illustrates a schematic diagram of an over current protection(OCP) circuit 200 according to one embodiment of the present invention.The OCP circuit 200 includes a comparator 201 and an OCP resistor 203.In one embodiment, the OCP circuit 200 can be used in a DC/DC converter.The OCP resistor 203 is coupled between the non-inverting terminal ofthe comparator 201 and ground.

In operation, the sensing current Isense flows through the OCP resistor203. Accordingly, a voltage Vocp equal to Isense*Rocp is produced acrossthe OCP resistor 203, where Rocp is a resistance of the OCP resistor203. Furthermore, a reference voltage Vref with a substantially constantvoltage level is provided to the inverting terminal of the comparator201. In one embodiment, the reference voltage Vref can be 400mv. Thecomparator 201 detects whether an over current condition occurs bycomparing the voltage Vocp with the reference voltage Vref.

In one embodiment, the over current condition occurs if the voltage Vocpreaches the reference voltage Vref, that is

Isense×Rocp=Vref.  (3)

Substituting equation (2) to equation (3), a current limit I_(LIM) ofthe load current I_(load) which incurs the over current condition can begiven by

$\begin{matrix}{I_{{LI}\; M} = {\frac{{Vref} \times {Rg}}{{Rocp} \times {Rsense}}.}} & (4)\end{matrix}$

The comparator 201 can output a signal OCP indicative of the overcurrent condition if the load current I_(load) reaches the current limitI_(LIM). According to equation (4), the current limit I_(LIM) isdetermined by the external components Vref, Rocp, Rg and Rsense. Due tothe configurability of these external components, the current limitI_(LIM) can be set or adjust to be compliant with various applications.

FIG. 3 illustrates a schematic diagram of a load line control circuit300 according to one embodiment of the present invention. The load linecontrol circuit 300 includes a remote sense amplifier RSA 301 and anexternal resistor 303. In one embodiment, the load line control circuit300 can be used in an electronic circuit to achieve relatively stableand accurate output impedance.

In one embodiment, the electronic circuit can be a DC/DC converter. Inthe example of FIG. 2, lines RFB+ and RFB− are coupled to the electroniccircuit using the Kelvin connection to receive a feedback signal whichrepresents a load voltage of the electrical circuit. The line RFB+ isfurther coupled to the non-inverting terminal of the remote senseamplifier RSA 301 through the external resistor 303. The line RFB− iscoupled to the inverting terminal of the remote sense amplifier RSA 301.The sensing current Isense indicative of the load current of theelectrical circuit is injected to the load line control circuit 300 viaa RSP pin. In one embodiment, the remote sense amplifier RSA 301 can beintegrated into an IC and interfaces with external components via theRSP pin and RSN pin. The output of the remote sense amplifier RSA 301can be compared to a reference signal, e.g., to control power deliveredto the load. The power delivered to the load can be limited or adjustedcontinuously or in steps.

In operation, a substantially constant voltage difference between pinsRSP and RSN can be achieved by a voltage positioning function inherentwith the load line control circuit 300. In this instance, if the loadcurrent changes, the voltage variation of the feedback signal (the loadvoltage) can be offset by the voltage variation across the externalresistor 303. Accordingly, the output resistance of the DC/DC convertercan be given by

$\begin{matrix}{{R_{loadline} = {\frac{\Delta \; V_{load}}{\Delta \; I_{load}} = {R_{LL} \times {{Rsense}/{Rg}}}}},} & (5)\end{matrix}$

where R_(loadline) is the output resistance and R_(LL) is the resistanceof the external resistor 303. According to equation (5), the outputresistance R_(loadline) is determined by the external resistors,resulting in a relatively accurate and stable value of R_(loadline).Furthermore, if the load current increases, the sensing current Isenseincreases according to equation (2) and the voltage across the externalresistor 303 will increase with the increased sensing current Isense.Accordingly, the output of the remote sense amplifier RSA 301 candecrease the load voltage to maintain the constant voltage differencebetween the pins RSP and RSN. As a result, power delivered to the loadof the electronic circuit is controlled. Furthermore, the load canconsume less power compared with the situation where the load current isincreased while the load voltage keeps the same. Therefore, the powerefficiency of the electrical circuit can be enhanced.

FIG. 4 illustrates a schematic diagram of a current monitor circuit 400according to one embodiment of the present invention. The currentmonitor circuit 400 includes an external resistor 401 and an externalresistor 403 coupled in series between a reference voltage source Vref1and ground. In one embodiment, the current monitor circuit 400 is usedin a DC/DC converter to provide a voltage V_(IMON) indicative of theload current at the IMON node. The voltage V_(IMON) can be given by

$\begin{matrix}{{V_{IMON} = {{{Vref}\; 1} = {\frac{Rb}{{Ra} + {Rb}} + {I_{load} \times \frac{Rsense}{Rg} \times \frac{{RaR}\; b}{{Ra} + {Rb}}}}}},} & \left. 6 \right)\end{matrix}$

where Ra is a resistance of the external resistor 401 and Rb is aresistance of the external resistor 403.

FIG. 5 illustrates a flow chart 500 of a method for current sensingaccording to one embodiment of the present invention. Although specificsteps are disclosed in FIG. 5, such steps are exemplary. That is, thepresent invention is well suited to performing various other steps orvariations of the steps recited in FIG. 5. FIG. 5 is described incombination with FIG. 1.

In block 501, a current flowing through a first resistor is converted toa voltage drop between positive and negative sides of the firstresistor. In one embodiment, the inductor current I_(L) flowing throughthe sense resistor 109 is converted to the voltage drop Vsense betweenpositive and negative sides of the sense resistor 109.

In block 503, the voltage at the positive side of the first resistor isapplied to a first pin of a sense amplifier. In one embodiment, thevoltage at the positive side of the sense resistor 109 is applied to theCSP pin of the sense amplifier 120, which includes the amplifier 103,the transistor 105 and the current mirror 101.

In block 505, the voltage at the negative side of the first resistor isapplied to a second pin of the sense amplifier through a secondresistor. In one embodiment, the voltage at the negative side of thesense resistor 109 is applied to the CSN pin of the sense amplifier 120through the external resistor 107.

In block 507, the sense amplifier employs a negative feedback to causethe voltage at the second pin to follow the voltage at the first pin. Inone embodiment, the transistor 105 is coupled to the amplifier 103 toform a negative feedback circuit, such that the amplifier 103 becomes avoltage follower to cause the voltage at the CSN pin to follow thevoltage at the CSP pin.

In block 509, a sensing current proportional to the current flowingthrough the first resistor is generated by the sense amplifier. In oneembodiment, the current mirror 101 included in the sense amplifiergenerates the current equal to I_(L)*Rsense/Rg to flow from thetransistor 105 to the external resistor 107. Due to the inherentduplication function of the current mirror 101, the sensing currentflowing through the output line 113 of the current mirror 101 is alsoequal to I_(L)*Rsense/Rg, which is proportional to the current I_(L)flowing through the sense resistor 109.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention,in the use of such terms and expressions, of excluding any equivalentsof the features shown and described (or portions thereof), and it isrecognized that various modifications are possible within the scope ofthe claims. Other modifications, variations, and alternatives are alsopossible. Accordingly, the claims are intended to cover all suchequivalents.

1. A circuit comprising: a first resistor that converts a currentflowing through said first resistor to a voltage drop between positiveand negative side of said first resistor; a second resistor coupled tosaid negative side of said first resistor; a voltage follower coupled tosaid positive side of said first resistor via a non-inverting terminaland coupled to said negative side of said first resistor through saidsecond resistor via an inverting terminal respectively to cause avoltage at said inverting terminal to follow a voltage at saidnon-inverting terminal; and a current mirror coupled to said voltagefollower to provide a sensing current proportional to said currentflowing through said first resistor.
 2. The circuit of claim 1, whereinsaid voltage follower comprises: an amplifier comprising said invertingterminal, said non-inverting terminal and a output terminal; and atransistor coupled between said current mirror and said invertingterminal, wherein a gate of said transistor is coupled to said outputterminal of said amplifier, and wherein a current flowing through saidtransistor is proportional to said current flowing through said firstresistor.
 3. The circuit of claim 1, wherein a ratio between saidsensing current and said current flowing through said first resistor isdetermined by resistances of said first and second resistors.
 4. Thecircuit of claim 1, further comprising: a third resistor that convertssaid sensing current to an input voltage; and a comparator, coupled tosaid third resistor, that generates a signal indicative of an overcurrent condition by comparing said input voltage with a referencevoltage.
 5. The circuit of claim 4, wherein a current limit incurringsaid over current condition is determined by said reference voltage andresistances of said first, second and third resistors.
 6. The circuit ofclaim 1, further comprising: a third resistor that converts said sensingcurrent to a voltage across said third resistor, wherein said sensingcurrent indicates a load current of an electronic circuit; and a remotesense amplifier, coupled to said electronic circuit through said thirdresistor, that stabilizes an output impedance of said electronic circuitbased on said voltage across said third resistor.
 7. The circuit ofclaim 6, wherein a variation in said voltage across said third resistoroffsets a variation in a load voltage of said electronic circuit tostabilize said output impedance.
 8. The circuit of claim 6, wherein saidoutput impedance is determined by resistances of said first, second andthird resistors.
 9. The circuit of claim 6, wherein said electroniccircuit comprises a DC/DC converter.
 10. The circuit of claim 6, whereina load voltage of said electronic circuit decreases in accordance withan increase in said voltage across said third resistor if said loadcurrent increases.
 11. The circuit of claim 1, further comprising: thirdand fourth resistors coupled in series between a reference voltagesource and ground; and a monitor node, coupled to a conjunction node ofsaid third and fourth resistors, that receives said sensing current andprovides a voltage at said monitor node indicative of a current at saidmonitor node.
 12. A system comprising: a DC/DC converter that provides aload current to a load; a first resistor, coupled to said DC/DCconverter, that senses said load current; a second resistor coupled tosaid first resistor; a voltage follower coupled to a positive side ofsaid first resistor via a non-inverting terminal and coupled to anegative side of said first resistor through said second resistor via aninverting terminal to cause a voltage at said inverting terminal tofollow a voltage at said non-inverting terminal; and a current mirror,coupled to said voltage follower, that provides a sensing currentproportional to a current flowing through said first resistor.
 13. Thesystem of claim 12, further comprising: an over current protectioncircuit that receives said sensing current and protects said DC/DCconverter from an over current condition based on said sensing current.14. The system of claim 12, further comprising: an output line controlcircuit that receives said sensing current and stabilizes an outputimpedance of said DC/DC converter based on said sensing current.
 15. Thesystem of claim 12, further comprising: a current monitor circuit thatreceives said sensing current and monitors a current at a monitor nodeof said DC/DC converter based on said sensing current.